Theory, Modeling and Simulation

Elementary excitations in liquids

May 15, 2013

The ratio of the Maxwell relaxation time and the time scale of configurational excitations (τM/τLC) plotted against temperature (normalized to the crossover temperature TA) for various models, including the results of the ab-initio MD simulation for Cu-Zr. Above TA, τM is approximately equal to τLC for various models, calculated here using different methods. This suggests the universality of the result.

Elementary excitations in metallic liquids were discovered through computer simulation, representing a major advance in the physics of liquids. In solids the elementary excitations of lattice dynamics are phonons, but in liquids they have a very short lifetime. The current work shows that the elementary excitations in liquids are not phonons but local configurational excitations (LCEs), in which an atom loses or gains one nearest neighbor. Molecular dynamics simulations with classical as well as quantum mechanical methods show that the Maxwell relaxation time, τM (= viscosity/shear modulus), is equal to the time-scale of LCEs, τLC, for a number of liquid metal alloys at high temperatures. Thus, these atomic-level excitations directly explain the macroscopic viscous behavior of a liquid. The current work also shows that the equality τLC = τM breaks down below a crossover temperature TA above which phonons are localized, and LCEs cannot communicate each other. This discovery will help understanding of the glass transition, which has long been a major mystery [see, e.g., P.W. Anderson, Science 267, 1615 (1995)].